In a wireless communication system, different categories of reference signals are used. For instance, one category of reference signals is used to estimate the channel which is needed to enable coherent demodulation of a received signal containing both control and data information. Another category is used for channel quality measurements to enable scheduling, link adaptation, power control etc. This category is not associated with received data or control.
For example, in the 3rd Generation Partnership Project 3GPP Long Term Evolution LTE (Release-8) uplink, reference signals which are used for coherent demodulation are referred to as demodulation reference signals, DMRS, and reference signals which are used for channel quality measurements are referred to as sounding reference signals, SRS.
In LTE, uplink sounding reference signals, SRS, resources follow periodic transmission and the related transmission parameters are configured semi-statically by higher layer radio resource control, RRC, signalling. The configured transmission parameters include the periodicity of SRS, the transmission sub-frame, bandwidth, cyclic shift of SRS sequence, transmission comb in the frequency domain and so on. On SRS, UE uses every second sub-carrier for transmitting the reference signal as known in the art. Thereby a comb spectrum is obtained. Related sub-carrier offset defines a transmission comb for the distributed transmission. The transmission comb provides another means to multiplex UE reference signals in addition to cyclic shifts. SRSes corresponding to different User Equipment devices (UEs or user terminals) are multiplexed by a code division multiple access scheme, CDM, or frequency division multiple access scheme, FDM: Each SRS is a cyclic shifted version of a cell specific base sequence or different transmission comb in the frequency domain. Different cyclic shifts are allocated to different users to make detection possible at the base station/NodeB.
LTE-Advanced Rel-10 is an evolution of the LTE Rel-8 system in which uplink multiple antenna transmission is supported. Concerning Sounding Reference Signals, SRS, for Rel-10, it has been decided to use antenna-specific and non-precoded sounding resources to support multiple antenna closed loop precoding. This will increase Sounding Reference Signal, SRS, utilization and interference among different User Equipments compared to Rel-8. (Precoding is in this context a coding scheme to control usage of the multiple antennas.)
In order to settle these problems and to bring higher flexibility to sounding resource allocation, it has been proposed to use Demodulation Reference Signals, DMRS, also for sounding in which case a UE is scheduled but does not transmit any Physical Uplink Shared Channel, PUSCH, payload data. In this case, the DMRS shall be non-precoded, to allow for antenna specific channel measurements.
Each Demodulation Reference Signal, DMRS, is a cyclic shifted version of a Zadoff-Chu cell-specific based sequence. Totally eight possible cyclic shifts can be allocated for different DMRSs. However in Rel-8, one of the cyclic shifts is used and the remaining seven cyclic shifts are not utilized.